Abstract: The tolerance of sodium alkyl carboxylic acid auxiliaries to hard water was evaluated through measuring the influence of auxiliary upon the adhesion of starch to cotton.The relation between the alkyl carbon chain of the auxiliary and the tolerance to hard water was examined.The relevant relations between the adhesion and electrolyte type as well as electrolyte dosage were investigated.The experiment results demonstrate that the existence of electrolytes in starch paste decreases the serviceability of sodium alkyl carboxylic acid,which means that the adhesion to fibers reduces.Different carbon chain length in sodium alkyl carboxylic acid auxiliaries gives different tolerance to hard water.The tolerance increases with the decrease in length of the carbon chain when the carbon atom number of the chain is within 8 to 18.